The actuating force of a wattmeter comes from the field of its current coil and the field of its
potential coil. The force acting on the movable coil at any instant (tending to turn it) is proportional to the
instantaneous values of line current and voltage.
The wattmeter consists of two circuits, either of which will be damaged if too much current is passed
through them. This fact is to be especially emphasized in the case of wattmeters, because the reading of
the instrument does not serve to tell the user that the coils are being overheated. If an ammeter or
voltmeter is overloaded, the pointer will be indicating beyond the upper limit of its scale. In the
wattmeter, both the current and potential circuits may be carrying such an overload that their insulation is
burning, and yet the pointer may be only part way up the scale. This is because the position of the pointer
depends upon the power factor of the circuit as well as upon the voltage and current. Thus, a low power-
factor circuit will give a very low reading on the wattmeter even when the current and potential circuits
are loaded to the maximum safe limit. This safe rating is generally given on the face of the instrument. A
wattmeter is always distinctly rated, not in watts but in volts and amperes. Figure 1-45 shows the proper
way to connect a wattmeter in various circuits.
Figure 1-45.A wattmeter connected in various circuits. TWO-PHASE SYSTEM
The watt-hour meter is an instrument for measuring energy. Since energy is the product of power and
time, the watt-hour meter must take into consideration both of these factors.
In principle, the watt-hour meter is a small motor whose instantaneous speed is proportional to the
POWER passing through it. The total revolutions in a given time are proportional to the total ENERGY,
or watt-hours, consumed during that time.
The following directions should be followed when reading the dials of a watt-hour meter. The meter,
in this case, is a four-dial type.